Abstract
The studies of charge transfer and fluorescence properties of single CdSe/ZnS quantum dots (QDs) in different environments are presented in this report. This report consists of two parts. In the first part we demonstrate diffusion-controlled electron transfer (DCET) model with regard to the blinking phenomenon and discussion several methods of blinking suppression of QDs. Recently, we observe that the blinking behavior of single QDs can be suppressed by embedding QDs in agarose gel. The absence of blinking suppression is not due to aggregation of QDs which was confirmed by antibunching experiments to demonstrate single-photon emission for the QD. Moreover, the long-time exponential bending tail from the power-law blinking statistics of single QDs could be significantly influenced by agarose gel concentration. We observed that an increase in gel concentrations accompanied with high bending rate. According to the DCET model of Tang and Marcus, the bending rate is related to the activation energy between the light and dark states of QDs. Since agarose gel has inherent negatively charged fibers, we suggested that the electron transfer rate between the light and dark states of QDs could be reduced or even blocked by changes in their electrostatic surrounding. In the second part we show that the photoluminescence and charge transfer of CdSe/ZnS QDs are pH-dependent. We presented experimental results of QDs imbedded in agarose gel fibers at different pH, and we also provided theoretical analysis of both the blinking behavior of single-QDs as well as the fluorescence intensity time trace from an ensemble of QDs. The combined approach of single-particle and ensemble measurements to investigate pH-dependent fluorescence properties of QDs has not been used previously. This study allows us to elucidate the electron transfer processes of QDs from the light state to the dark state. We have also estimated from the experimental data from both single-particle and ensemble measurements the free energy gap and the reorganization energy for this system. The observation demonstrated that the activation energy for the charge transfer and the free energy gap between the light and the dark states increased with an increase in the concentration of H+ ions. Therefore, we proposed that the electron transfer in QDs in agarose gel occurs in the Marcus inverted regime.